add supports_training_losses() trait

Author: ablaom

src/MLJModelInterface.jl

@@ -22,7 +22,8 @@ const MODEL_TRAITS = [
     :hyperparameters,
     :hyperparameter_types,
     :hyperparameter_ranges,
-    :iteration_parameter]
+    :iteration_parameter,
+    :supports_training_losses]
 
 # ------------------------------------------------------------------------
 # Dependencies (ScientificTypes and StatisticalTraits have none)

src/model_api.jl

@@ -33,6 +33,9 @@ implement this method to return an `AbstractVector` of the losses
 in historical order. If the model calculates scores instead, then the
 sign of the scores should be reversed.
 
+The following trait overload is alse required:
+`supports_training_losses(::Type{<:M}) = true`
+
 """
 training_losses(model, fitresult, cache, report) = nothing